Emissions sensor technology needs marinising

Upcoming regulations from the IMO and the Environmental Protection Agency (EPA) have placed emission reduction and control at the top of the marine industry’s agenda. Meeting regulations is a driving force for shipping companies and engine builders and extensive development resources are allocated in order to achieve compliance with IMO Tier II and Tier III. For the optimisation of fuel efficiency and documentation of emissions, stack emissions monitoring will be an integrated part of the propulsion system of tomorrow.
The emission of CO2 is directly proportional to fuel consumption. The CO2 emission will normally increase as NOx, SOx and PM reduction solutions or abatement technologies are implemented. In other words, exhaust gas emission monitoring becomes more and more important to meet tighter emission regulations (including regulations on NOx and SOx) and to optimise fuel consumption at the same time.
With more than 2,000 smoke density monitors (PM-opacity) and more than 50 NOx analysers installed on vessels worldwide, Green Instruments believes that equipment for emissions monitoring must be simple, reliable, easy to operate, cost effective, and, furthermore, it must require little maintenance and provide a quick response.
Most of the existing emissions monitoring systems for the marine market are based on technology that has been developed and used for land-based industrial applications. The primary technologies used for marine applications are chemiluminescence (CLD), non-dispersive infrared (NDIR), laser-based and zirconium dioxide (ZrO2). All these technologies have both advantages and disadvantages.
A common feature of these technologies is that they have proven success in land-based applications and that success rests on the fact that there is no vibration and that the exhaust gasses stem from fuels that are significantly cleaner than marine fuels. The fact that the ambient conditions are less challenging than in the marine environment plays a large role.
Chemiluminescence sensors are currently the reference measuring method for NOx in the NOx Technical Code 2008. Chemical sensors, chemiluminescence and zirconium dioxide, are well known, but have a major disadvantage because they have to be exposed to the exhaust gas and will be influenced by the interaction with aggressive components and particles.
Chemiluminescence sensors are exposed to the exhaust gas via a sampling and conditioning system since the sensor elements require dry and cool samples. This process of drying and cooling will change the composition of the sample due to quenching. Furthermore, any leakage in the sampling lines, that can have lengths of as much as 50m, will impact the gas composition. The samples are drawn to the analyser by partial vacuum and thus any leakage would dilute the sample with atmospheric air. Both effects can be compensated for or minimised on well-maintained industrial installations. On ships, however, with ever smaller crews and chief engineers having to take care of a multitude of systems, poor maintenance can easily invalidate measurements.
Zirconium dioxide sensors are a proven technology within the automotive industry. So far the decisive drawback of the ZrO2 technology is that it can only be used to measure NOx and O2. The automotive industry does not use fuels with sulphur contents as high as those in most marine fuels. Sulphur in exhaust gas tends to cause deterioration in the chemical composition of the zirconium dioxide sensor. This can be alleviated by frequent replacement of the sensor element. Although frequent replacements are not to the liking of purchase managers or chief engineers, this is, for the time being the technology with the lowest maintenance cost and lowest total cost of ownership.
The optical sensors, including NDIR sensors and laser based sensors, have a major advantage since there is no chemical interaction between light and exhaust gas. This allows for a semi-in-situ measurement where the analyser is mounted on the smokestack. However, working with non-conditioned samples creates a common challenge in keeping the exhaust gas separated from both the source of radiation and the detector, while keeping the barrier clean enough to allow passage of radiation through the gas and to the detector.
Traditional NDIR measurements are based on the principle that non-symmetrical molecules absorb infrared energy at different wavelengths and have different levels of absorption depending on the specific molecule. The measurement is established by exposing a gas to infrared radiation over a given path length and then measure the level of radiation energy after it has passed through the gas. Passing through the gas, the IR radiation will be partly absorbed and the absorption will be different at different wavelengths according to the specific absorption spectrum of the particular gas and its concentration. The level of radiation is measured simultaneously at two different wavelengths. At one wavelength there is a significant absorption and at the other there is a very low absorption. These are often referred to as ‘suspect and reference’. By comparing the suspect and reference results it is possible to calculate the concentration of a particular gas ‘all things being equal’.
The traditional infrared measurement method depends on ‘all things being equal’ because the measurements take place at different wavelengths and are not correlated. According to the Wien-Planck law, any change in temperatures will have a different impact on suspect and reference so that the measurement is no longer valid. The dependency of ‘all things being equal’ is precisely the Achilles heel of infrared measurement because all things are never equal. The infrared source will age over time and change spectral characteristics. The temperature of the gas or measurement chamber might vary over a measurement period. The measurement chamber might be contaminated and change the optical properties. When all things are not equal, traditional IR measurement cannot deliver reliable data. So in order to circumvent these laws of physics, traditional infrared measurement is dependent on frequent recalibration, keeping temperatures extremely stable, avoiding contamination of the measurement chamber etc. In the marine environment, these measures translate into additional equipment for cleaning and conditioning samples, having complex additional contractions for maintaining a stable temperature, and in most cases also for transporting samples from the smokestack to the engine control room in order to maintain stable and cool environments for this equipment.
Furthermore, the sensors have a broad band radiation and in order to narrow the bandwidth, in many infrared sensor systems, gas filter correlation is used. This technology requires quite delicate rotating filters in the analyser. Given the nature of the marine application, it will be a disadvantage since the analyser will be exposed to vibration when mounted on a smokestack. All of these measures add significant cost to the installation and operation of traditional infrared measurement equipment.
Laser based equipment has the advantage of an extremely low bandwidth of radiation and thus lasers are able to target very specific absorption wavelengths. However, the nature of a laser beam makes them susceptible to contamination of barriers at source and detector, since the beam covers an extremely low area when passing through the barrier. Because the laser based equipment typically works at extremely low bandwidths, and the degree of absorption at specific wavelengths is pressure dependent, the optimal wavelength for measuring the absorption varies. Due to this, laser based equipment relies on pressure compensation in order to establish a measurement at the right wavelength.
Some of the difficulties of applying optical sensors on-stack can be remedied by integrating them into a sampling and gas treatment system. This will however add all the drawbacks of these latter systems as mentioned above.
The challenges of using optical sensors (including both infrared sensors and laser based sensors) in unconditioned exhaust gas measurements are mainly temperature and pollution, i.e. distortion of the optical pathway. Optical barriers – sight glasses – may be used to shield the sensitive sensor components and electronics from the exhaust gas components. These sight glasses have to possess special properties such as high optical transmissivity in relevant infrared wavelength ranges and a dedicated self-cleaning surface to avoid film formation and particle contamination, as well as thermal, mechanical, and chemical long term stability in the environment.
Furthermore, the electronics must be protected from the heat from the unconditioned gas that can reach up to 350°C. The sensor as a whole must be constructed in such way that it can withstand vibrations and elevated ambient temperatures in the smoke stack environment.
We can see the development trend in the mechanical construction of the sensors, so that infrared sensors, or especially laser based sensors, with all of theirs strengths can be used. The mechanical construction of the emissions monitoring system needs to share the non-complex construction of a chemical sensor system with no moving parts. This will facilitate the sensors, with no direct contact with the exhaust gas. With neither moving parts nor delicate construction it will therefore be very robust under vibration conditions.
In order to respond to gradually increasing regulation and demand for monitoring different gases, the sensor system can be flexibly adapted to different measurement and application requirements. This means that the sensor system with basic functionality for measuring CO2 and NOx can be extended to measure, in addition for example, SO2, NH3 and other gases.
The marine industry has to face the upcoming requirements for measuring emissions for the purposes of documentation and engine optimisation. The current technologies are challenged by the fact that they are based on technologies that have been developed for land-based applications with a different ambient environment and a different composition of the exhaust gas. The marine market needs an emissions monitoring system that can be implemented as a natural part of operating the ship and its propulsion system, and it must be designed to operate under the extreme conditions on board a vessel. Robustness, low maintenance and reliable measurements are crucial requirements. When this type of technology is available, the benefits of implementing emissions data monitoring in the operations of modern shipping will heavily outweigh the cost.
Source: Motor Ship